The auction of the 700 MHz spectrum by the Federal Communications Commission (FCC) resulted in part from the shift of television broadcasting from analog to digital service. Some of the new license holders have begun rollout of a Digital Video Broadcast to Handheld (DVB-H) mobile television (TV) entertainment service. Since receivers for this service may be expected to be integrated into cell phones and similar devices, circularly polarized broadcast signals will likely be preferred.
By providing a signal with horizontal and vertical components of comparable strength, circular polarization offers independence between receiving antenna orientation and reception, at least within a plane perpendicular to a line of propagation between the transmitting and receiving antennas. That is, a simple (linearly polarized) receive dipole is capable of receiving, and is substantially insensitive in orientation with respect to, a circularly-polarized broadcast signal. By contrast, with a vertically (linearly) polarized transmitted signal, the same receive dipole receives very little signal if placed horizontally, and likewise for a horizontally polarized signal and a vertically oriented receive dipole. This can be a significant consideration in ensuring robust and stable received-image quality in a mobile handheld imaging device, for example. Multipath issues, such as reflections from buildings that can reverse polarization handedness and delay time-critical signals, are often managed through signal processing.
Omnidirectionality is frequently a desirable attribute of broadcast antennas, particularly in view of long-established FCC preference for azimuth uniformity in consumer-oriented broadcasting. A fundamental omni radiator, well understood in the art, is a vertical dipole (or a ground-plane-mirrored monopole), that cannot provide circular polarization and is limited regarding power, gain, beam tilt, and null fill. Some previous omni designs, such as that disclosed in U.S. Pat. No. 6,441,796 ('796), issued Aug. 27, 2002, incorporated herein by reference, can provide circular polarization.
In antennas according to the '796 patent, a plurality of omni radiators (bays) are configured in a vertical array. Each radiator in the '796 patent includes two or four arcuate, rod-section dipoles lying on quasi-helical paths around a vertical axis of the antenna common to all bays. As used herein, the term “quasi-helical” describes a radiator formed from material having a suitable shape, such as a cylindrical rod, effectively wrapped into a planar arcuate shape, then rotated without further forming to an orientation approximating a helical path. A projection into a plane perpendicular to the vertical axis of the antenna of a quasi-helical radiator is elliptical; a true helical radiator has a circular projection into that plane. A rod formed into true helical form also does not lie in any plane. The effect of using a quasi-helical radiator is to broaden the impedance bandwidth of the antenna compared to a true-helix equivalent.
The dipoles in the '796 patent are each driven near one end of one monopole, with the centermost ends of the monopoles (the midpoints of the dipoles) grounded to conductive radial structural components. A central hub of each bay is mounted to a strut; the struts project laterally with selected vertical spacing from a vertical bearing structure. Such a configuration is readily applied to a side-mounted antenna on a tower, for example.
The radiative parts of antennas according to the '796 patent emit a signal having a specific circular polarization in accordance with their arrangement—for example, a mirror-image arrangement (opposite direction of advance of the helical paths of the dipoles) would produce opposite circular polarization.
In many other previous omnidirectional antenna designs, individual circularly-polarized radiators are strongly directional. For a multiple-bay antenna using directional radiators to broadcast with a reasonable approximation of azimuth uniformity, three or more separate radiators in each bay are needed, pointing radially outward around a vertical axis. The radiators can be mounted around a central member for top mounting, i.e., mounting of the antenna at the top of a structure. Antennas including such elements require more radiating devices and more power distribution devices than do intrinsically omnidirectional radiators.
In addition to circular polarization, increasing transmitter power output to 5 KW is planned under the new bandwidth assignments in order to achieve effective radiated power (ERP) that approaches the FCC-permitted maximum. This power level is high compared to that of S-band transmitting systems currently used for purposes similar to those for which the auctioned upper-L band spectrum is intended. The new requirements also call for an economical antenna solution and a compact equipment package, both highly desirable attributes for implementation of a nationwide infrastructure. Small size in combination with a simple physical arrangement may result in low wind loading. Other considerations include capability to use a single product over the entire new spectrum without alteration, or to combine multiple signal channels on a single antenna.